In a radiotelephone system, the calling area is divided into cells, each of the cells having a base station which is in communication with a multiplicity of mobile stations located within the cell. Each of the mobile stations includes a radiotelephone. In each cell, control signals are sent from the base station to the mobile stations to accomplish functions such as an assignment of a frequency band for telephonic communication and initiation of telephonic communication. The control signals are transmitted by analog control channels (ACC) and/or by digital control channels (DCCH). By example, in the case of a dual mode system, such as the one specified by IS-136, the dual mode capable mobile station is able to receive and respond to either an ACC or a DCCH.
An aspect of the design of a radiotelephone system is the size of a cell surrounding a base station, specifically with respect to the range of an ACC and the range of a DCCH transmitted by the base station. Near the outer edge of a cell, it is necessary for the ACC and/or the DCCH to have sufficient signal strength to enable a mobile station to respond to a page (a request to respond to an incoming telephone call). The footprint or cell size of an ACC with equal field strength to the DCCH is larger because the ACC signal tolerates more co-channel interference. This is due primarily to the simpler modulation scheme and larger error margins of the ACC, as compared to the DCCH. As such, a system based on the use of the ACC has an inherently larger working range than a system that uses a DCCH with comparable layout rules.
The use of the DCCH is typically preferred in that, since the capacity of the DCCH is significantly higher than the capacity of the ACC, the network operator can minimize the number of, and costs associated with, the control channels. The DCCH also offers features not found on the ACC, such as a capability to provide Short Message Service (SMS), and the use of the DCCH furthermore tends to extend mobile station battery life.
Consequently, it is advantageous for a mobile station to switch from an ACC to a DCCH when there is adequate signal strength to do so. Also, it is advantageous for a mobile station to switch from a present DCCH, which may have faded to a weakened signal strength, to a stronger DCCH.
A problem arises in that, in presently available mobile stations, a loss of pages or other control signals may occur since the mobile station will stay tuned to the control channel until the signal degrades to the point that it is no longer useable. Furthermore, and at least for the case of IS-136, information is transmitted on the DCCH for identifying a channel number of a DCCH (if any). However, if the signal has degraded the mobile station may be unable to receive this DCCH information. It is also useful for the mobile station to periodically measure the signal strength of the DCCH radio channel, and also other DCCH channels.
U.S. Pat. No. 4,872,204, to Hammer et al., discloses the operation of a radio communication system including a central station and a plurality of individual remote stations. A determination is made as to whether or not a control signal received at a remote station from a central station is indeed intended for that remote station. If not, and for the remainder of the transmission, the remote station operates so as not to receive control signals from the central station to which it is assigned.
Commonly assigned U.S. Pat. No. 5,471,655, to Raimo Kivari, discloses an extended standby mode of operation for a radiotelephone. Kivari's invention provides a method, and apparatus for performing the method, for reducing the power consumption of a radiotelephone. The method includes steps of (a) receiving with a receiver of a radiotelephone at least a Word Synchronization pattern from a frame that is transmitted over a control channel; (b) receiving a first repeat of a control message that follows the Word Synchronization pattern within the frame; and (c) determining if the first repeat was correctly received. If the first repeat was correctly received, the method includes a further step of (d) determining if at least one reception-related criteria is met. If the reception-related criteria is determined to be met, the method includes a further step of (e) selectively removing operating power from one or more portions of the radiotelephone receiver for a predetermined time period during a remainder of the frame.
The at least one reception-related criteria can be a Message Error Rate (MER) that indicates a number of correctly received frames out of a total number of n last received frames. The reception-related criteria may also include a difference value between a minimum Received Signal Strength Indicator (RSSI) value and a maximum RSSI value that are detected during a predetermined interval of time. For this latter case, the at least one reception-related criteria is determined to be met if the MER is below a predetermined first threshold, and if the RSSI difference value is below a predetermined second threshold.